The coatings industry is undergoing massive changes in its philosophy, a great deal thereof being the result of the ecological problems associated with the use of the conventional solvent-based coating compositions. In the past the most commonly used coatings were compositions of pigmented or unpigmented vehicles that were diluted with solvents to facilitate application. After application on the surface, the solvents were permitted to evaporate, often into the atmosphere, leaving a dry, uniform coating on the surface. With time, advances led to the development of organosols, plastisols, emulsions, dispersions, 100 percent reactive solids coatings, water-borne coatings, all of which are used today to some extent. In addition, within the past two decades much effort has been expended in the preparation and application of powder coatings, the initial impetus therefor being the discovery of the fluidized-bed technology in Germany. This initial effort in the powder coating field involves the submersion of a preheated article into a bed of powder particles, the particles generally being from 50 to 150 microns in size, fluidized by a stream of gas. On immersion in the fluidized-bed, powder particles adhere and coalesce on to the surface of the article forming a uniform coating, usually in excess of 5 mils thick and often as much as 100 mils thick. The relatively large particle sizes required for fluidized-bed coating technology precludes the formation of the thin coatings of from 0.2 to 2 or 3 mils thick that are often desired. Many further attempts have been made to apply such thin coatings, the most recent efforts being directed to the electrostatic spray coating of particles to the grounded article to be coated followed by heating to fuse the particles and form a uniform coating. This method has recently been finding wide acceptance because of the many advantages associated therewith, e.g., improved film properties, absence of volatile solvents, elimination of water pollution problems, elimination of preheat of the substrate, essentially 100 percent utilization of material, reduced capital expenditures, ease of changeover. However, many difficulties have been encountered in producing the powders required for this use since the conventional grinding and spray drying procedures heretofore used have not been capable of the production of powders having the desired physical properties that would enable one to obtain thin films by the electrostatic spray coating procedures. In the past difficulties have been experienced in producing particles of proper shape having the desired fine particle size and the desired particle size distribution required to obtain the desired film thicknesses. Previous processes include dry blending, melt processing, wet processing, spray drying and combinations and modifications of these.
One area of special interest for thin film powder coatings is the area of vinyl halide polymers. However, experience has shown that vinyl halide polymers, such as poly (vinyl chloride) and copolymers thereof, generally cannot be applied in thin film coatings from powders because such polymers cannot be economically ground to the fine sizes required for electrostatic spray application and generally powder coatings thereof do not have the physical properties and appearance desired.